This invention relates to an apparatus for and a method of measuring intraocular pressure in the human eye. This invention more particularly relates to an applanation tonometer comprising electrical sensor circuitry for measuring such intraocular pressure in the human eye.
It is well known that excessive internal pressure within the human eyeball is a component of glaucoma, a disease of the eye. This disease accounts for a significant percentage of all blindness. Surveys have shown it to be present and unrecognized in a significant number of people, particularly people over the age of 40 and even more so for people over the age 50.
It is also known that where the presence of glaucoma can be identified at an early stage, damage to the eye and subsequent blindness can be arrested. Appropriate medication and surgery can serve to arrest the progress of the disease so that useful vision is retained.
In view of the fact that glaucoma is widespread, numerous proposals have been made for measuring the internal eyeball pressure. Many of these are complex precision instruments, which are expensive, and which require elaborate clinical settings for their operation. Typically, such instruments apply an amount of force to the eyeball, sufficient to allow an objective measurement of specific flattening (applanation) or indenting (indentation) of the surface of the eye. The amount of force required to achieve a certain applanation or indentation is correlated with the intraocular pressure measured internally, and usually expressed in mm of mercury.
Conventionally, the clinical instrument involved has some element which is applied directly to the cornea of the open eye to measure the applanation or indentation of the cornea. In view of natural human reflexes, this requires a topical anesthetic. The equipment is complex and costly and requires a trained and sophisticated technician to operate it. Other proposals have been made, and the following patents list proposals known to the applicant: U.S. Pat. Nos. 1,637,421; 1,661,718; 2,656,715; 5,176,139; and 5,197,473; French Patent 2,542,603; and Russian Federation Patents 2,004,187 and 457,466.
The Lipschutz U.S. Pat. No. 1,637,421 is a pressure indicator. It is not concerned with measuring eyeball pressure, but rather it is concerned with applying pressure to other parts of the human body. It is based on the well known phenomenon that sensitivity to pressure of an area of the body is an indication of disease. More particularly, it relies on the fact that the progress of the disease is related to the sensitivity of an associated area of the body. As such, it provides a device enabling the pressure applied to a particular area to be measured, so this pressure can be correlated with the progress of the disease. No clear directions are given, with regard to applying this technique to the human eye. Measuring pressure in the human eye presents unique and difficult problems, as compared to other parts of the anatomy. As the human eye is sensitive and delicate, everyone has a strong, natural reflex to close their eyes, if any attempt is made to touch the eye. This Lipschutz patent does not address this issue.
A hardness testing device is disclosed in U.S. Pat. No. 1,661,718 which is of marginal relevance.
An ocular tension indicator is disclosed in the Tolman U.S. Pat. No. 2,656,715. However, this requires the eyeball to be contacted. It relies upon relative axial displacement of different components of known, set weight, to determine the pressure within the eye. As such, it appears to be a delicate, precision instrument. Since it must contact the naked eye, it cannot be used outside of a clinical setting.
The two Fedorov U.S. Pat. Nos. 5, 176,139 and 5,197,473 disclose an ocular tonometer and a related method. This relies on a somewhat unique technique where a ball is permitted to fall freely onto an eyelid-covered cornea. The kinetic energy of the ball deforms a cornea. The amount of the ball rebound varies depending upon the amount of intraocular pressure and this is judged against the height of the ball rebound. This technique would appear difficult to carry out, since it depends upon judging the height of the ball rebound.
Russian Patent 457,466 discloses an intraocular pressure transducer. This relies upon a Hall effect generator. Weights determine the penetration force of a plunger, whose displacement is sensed by the Hall effect generator with an output proportional to the displacement. Russian Patent 2,004,187 discloses an eye tonometer having a hollow cylindrical body with tips and working end face surfaces. It is not clear how this device is intended to work. In any event, it is again intended to be applied to the naked eyeball, which again would require the application of a topical anesthetic in a clinical setting.
Now, one of the problems with measuring intraocular pressure is that it can vary during the course of the day, and even from hour to hour. Accordingly, it is highly desirable to provide some simple, inexpensive technique for measuring this pressure. This technique should enable an ordinary person to measure the intraocular pressure within their eyes, without requiring complex expensive equipment, without requiring attendance at a clinic or the like, and without requiring the time of highly trained clinical staff.
An applanation tonometer comprising: a main body; a plunger mounted in the main body for movement relative thereto and having a first end portion comprising a contact member for contacting an eyelid, the contact member being substantially planar and sufficiently large that, in use, the eyeball is flattened and subjected to applanation, and the plunger having a second end portion mounted in the main body; a transducer mounted between the plunger and the main body, for converting a load applied to the first end portion of the plunger into an electrical signal; an electrical display device connected to the transducer, for indicating the magnitude of the load applied to the first end portion of the plunger; wherein the plunger is slidably mounted within the main please send your reply to body; wherein a spring member is mounted between the plunger and the main body for providing a spring biasing force in relation to the displacement of the plunger relative to the main body, wherein the transducer includes a sensing device attached to the plunger, for conversion of the displacement of the plunger into the electrical signal, wherein the sensing device comprises an elongate member provided with a plurality of slot regions, and a plurality of blocking regions, alternating with one another, and wherein the transducer includes measurement means provided at a measurement location, for counting blocking regions and slot regions passing by the measurement location.
In accordance with another aspect of the present invention, a method of obtaining information concerning the pressure within the eyeball of the subject, is obtained by determining the load on the eyeball when a pressure phosphene is detected by the subject. The method further includes applying the load to the eyelid, so as to apply pressure through the eyelid to the eyeball. The load applied to the eyelid is progressively increased until the pressure phosphene is detected, terminating increasing the load when the pressure phosphene is detected, and noting the largest load applied to the eyelid as the load causing onset of the pressure phosphene.
The method of obtaining the load applied to the eyelid includes providing a tonometer including: a main body; a plunger slidably mounted within the main body and including a contact member at one end for contacting in the eyelid; a sensing device attached to the plunger and extending into the main body; an electrical measurement apparatus for detecting displacement of the sensing device; a spring member acting between the plunger and the housing, biasing the contact member away from the housing; a reset switch for resetting a previous reading indicative of the load; and an electrical display for displaying the applied pressure detected by the tonometer, the method comprising:
(a) resetting the tonometer by depressing the reset switch;
(b) locating the contact member of the plunger against the eyelid;
(c) displacing the main body of the tonometer towards the eyelid so as to displace the plunger into the main body against the action of the spring member, whereby the electrical measurement apparatus determines the displacement of plunger and the pressure detected by the tonometer;
(d) when the subject notices the pressure phosphene, terminating displacement with the main body and removing the tonometer; and
(e) reading the applied pressure magnitude from the display.